1. Field of the Invention
The present invention relates to dot matrix line printers, and more particularly to printers of that type in which a plurality of hammers mounted along the length of a reciprocating shuttle are selectively released in synchronism with the constantly changing position of the shuttle to print dots on an adjacent print paper.
2. History of the Prior Art
Dot matrix line printers are known in which a plurality of hammers mounted along the length of a shuttle which undergoes reciprocating motion relative to a length of print paper are selectively fired to print dots on the paper. An example of such a printer is provided by U.S. Pat. No. 3,941,051 of Barrus et al., PRINTER SYSTEM, which patent issued Mar. 2, 1976 and is commonly assigned with the present application. The printer described in the Barrus et al. patent includes an encoder which generates "fence post" pulses in response to movement of the printer shuttle past a succession of generally equally spaced positions along the stroke of the shuttle. The fence post pulses provide a representation of the actual position of the shuttle relative to the print paper and are used in the timing of hammer firing.
In printers of the type described in U.S. Pat. No. 3,941,051 of Barrus et al. in which the printing is accomplished in dot matrix fashion, the various dot positions across the print paper can be referenced to the fence post pulses so that the fence post pulses can be used to time hammer firing. To do this, each dot position is related to the fence post pulse which occurs immediately prior thereto during movement of the shuttle. The time lapse between the immediately prior fence post pulse and the dot position can be related to the known hammer flight time so as to arrive at a hammer firing point which is related to one of the fence post pulses and which will produce printing of a dot at the dot position. The hammer flight time is the known time lapse between initiation of hammer firing and actual impact of the hammer with the paper.
It is known to provide printers of the type described with a variable dot density. In such instances the circuit for generating the hammer fire pulses must be capable of referencing the different dot positions corresponding to the different dot densities to the fence post pulses so that the hammer fire pulses can be varied as necessary in accordance with each change in dot density. An example of an arrangement for accommodating different dot densities in this fashion is provided by U.S. Pat. No. 4,415,286 of Jennings, VARIABLE PRINT DENSITY ENCODER SYSTEM, which patent issued Nov. 15, 1983 and is commonly assigned with the present application. As described in the Jennings patent, a stored initial offset value is used to initially establish a desired phase relationship between the hammer fire pulses and the fence post pulses. Thereafter, a stored pulse interval value is used to generate the hammer fire pulses at the desired frequency. The desired phase relationship is maintained by measuring the time distance between selected ones of the hammer fire pulses and the preceding fence post pulses, comparing the measured time interval with a stored value representing the desired offset and applying any difference as an error signal to alter the time interval between the immediately following pair of hammer fire pulses.
In the printer described in th previously referred to U.S. Pat. No. 3,941,051 of Barrus et al., the shuttle is driven in reciprocating fashion by a rotating cam which continuously engages the pulley of a spring-loaded cam follower attached to the shuttle. This provides the shuttle with a trapezoidal velocity profile. Each stroke of the shuttle is characterized by the rapid acceleration thereof in generally linear fashion to a relatively constant nominal velocity which is maintained during most of the stroke. At the end of the stroke the shuttle decelerates to rest rapidly and in generally linear fashion. Because the shuttle moves at the relatively constant nominal velocity during a substantial portion of each stroke, printing can be confined to the constant velocity region for most applications of the printer. Moreover, even if printing were carried out during acceleration and deceleration of the shuttle, the timing of hammer firing could be done in reliable fashion because of the predictable nature of the cam drive and the relatively precise shuttle velocity profile which can be assumed therefrom.
The problem of providing reliable hammer fire signals becomes more serious where the shuttle is driven in reciprocating fashion by arrangements which do not employ a cam drive. For example, in the previously referred to U.S. Pat. No. 4,415,286 of Jennings, the shuttle and an associated counterbalancing element are disposed on the opposite sides of a pair of spaced-apart rotatable pulleys so as to function as a linear motor. An arrangement of permanent magnets and coils drives the shuttle and the counterbalancing element in reciprocating fashion with the counterbalancing element or the shuttle or both rebounding from resilient elements such as springs to effect the rapid turnaround thereof.
A similar linear motor arrangement is described by U.S. Pat. No. 4,463,300 of Mayne et al., LINEAR MOTOR DIGITAL SERVO CONTROL, which patent issued July 31, 1984 and is commonly assigned with the present application. Such arrangements drive the shuttle in accurate and controlled fashion during the constant velocity portion of each stroke of the shuttle assembly. However, during each turnaround in which the shuttle is decelerated to rest, reversed in direction and then accelerated back up to the constant nominal velocity, relatively little control is exercised over the arrangement so that the precise behavior of the shuttle during turnarounds is difficult to predict. The Mayne et al. patent, for example, describes an arrangement for driving the shuttle through the turnaround which applies a single drive signal at the start of each turnaround. The drive signal is continuously updated in accordance with the constantly changing characteristics during the turnarounds.
Compounding the problem of accurate shuttle control and the generation of accurate hammer fire pulses in shuttle drives of the type referred to in the previously referred to Jennings and Mayne et al. patents is the fact that such linear motor arrangements have a velocity profile which is more sinusoidal in nature than in the case of a cam driven shuttle and in which the constant velocity portion of each stroke is somewhat shortened. This reduces the portion of each stroke over which printing can be performed unless the printing region of the stroke is extended into the acceleration and deceleration portions thereof The timing of hammer firing during acceleration and deceleration is not a major problem so long as the velocity as well as the position of the shuttle are closely monitored. However, such close monitoring typically requires rather complex circuitry to achieve. Alternatively, the velocity characteristics of the shuttle during acceleration and deceleration can be approximated by the storage and continuous use of permanent values representative thereof. This frequently results in timing errors, however, because of the lack of close control in such systems which do not employ a constantly engaged cam and in which the rebounding mechanisms and other components may vary with time so as to change the velocity characteristics of the shuttle in the regions of acceleration and deceleration.
Accordingly, it would be advantageous to provide an improved print hammer timing system for generating hammer fire pulses. It would furthermore be advantageous to provide such an improved system in which shuttle velocity during acceleration and deceleration can be predicted with reasonable accuracy and without the need for complex circuitry. It would still furthermore be advantageous to provide an adaptive print hammer timing system in which stored values representing shuttle velocity are periodically updated at a rate which takes into consideration changes in the gradually varying characteristics of the shuttle drive system.